OFDM is a promising technique for supporting cooperative transmission in mobile ad hoc networks (MANETs). To achieve a large diversity gain for combating fast time-varying and frequency-selective fading as well as tolerating imperfect synchronization among mobile nodes, we propose an asynchronous cooperative transmission scheme using distributed unitary space-frequency coded OFDM (USFC-OFDM), which has a lower decoding complexity with no need for any channel estimation and can provide better reliability and robustness when nodes move fast. A TDMA-based cooperative medium access control (CMAC) mechanism and an on-demand cooperative routing protocol are also proposed to support the USFC-OFDM scheme. Through simulation results, we show that the proposed USFC-OFDM scheme can significantly improve the network performance in terms of packet delivery ratio and bit error rate in highly dynamic MANET environments.